The present invention relates to a cathode active material including a lithium complex oxide, a method of manufacturing the same, and a battery.
In recent years, since portable devices such as notebook computers and cellular phones have become more sophisticated and multifunctional, power consumption of the devices has been increasing, and a further increase in capacities of batteries as power sources for the devices has been demanded. Among batteries, in terms of cost effectiveness and a reduction in size and weight, a demand for secondary batteries with a higher capacity is great. As a battery meeting such a demand, for example, a lithium secondary battery is cited.
A lithium secondary battery which is now commonly used uses lithium cobalt oxide for a cathode and a carbon material for an anode, and its operating voltage is within a range from 4.2 V to 2.5 V. In the case of a lithium secondary battery operating at 4.2 V at the maximum, the lithium secondary battery uses only approximately 60% of the theoretical capacity of a cathode active material such as lithium cobalt oxide used for a cathode. Therefore, when a charge voltage is further increased, a remaining capacity can be used in principle, and in reality, it is known that when a voltage at the time of charge is 4.25 V or over, a higher energy density is achieved (refer to Patent Literature 1).
However, when the charge voltage is increased, the potential of the cathode is increased, so an oxidation atmosphere near the cathode is increased, thereby an electrolyte is easily deteriorated by oxidative decomposition. As a result, issues such as a decline in charge-discharge efficiency and a decline in cycle characteristics arise. Moreover, such a reaction is more severe at high temperature, so there is an issue that deterioration of the electrolyte at the time of operating or storing the secondary battery at high temperature is pronounced. Further, in some cases, the secondary battery is used under a low-temperature environment such as cold climate, so a secondary battery having superior characteristics not only at high temperature but also at low temperature is desired.
To improve characteristics, a method of forming a solid solution of an element such as aluminum (Al), magnesium (Mg) or titanium (Ti) in a lithium complex oxide such as lithium cobalt oxide is conventionally well known. Moreover, as a technique to improve stability or low-temperature characteristics of a cathode active material, a method of forming a coating film made of a stable material on a surface of an active material is cited. For example, Patent Literature 2 describes that a surface of lithium cobalt oxide is coated with aluminum oxide (Al2O3), and Patent Literature 3 describes that an aluminum-containing layer is formed on a surface of a nickel-cobalt complex oxide. Further, Patent Literature 4 describes that a surface of lithium cobalt oxide is coated with lithium titanate (LiTiO2), and Patent Literatures 5 and 6 describe a method of forming a surface layer made of a compound represented by MXOk (M represents metal, X represents an element capable of forming a double bond with oxygen, and k=2 to 4). Moreover, Patent Literature 7 describes that the concentration of manganese (Mn) on a surface of lithium oxide is higher than in the interior of lithium oxide, and Patent Literature 8 describes that a particle surface of lithium cobalt oxide is coated with a sulfate.
[Patent Literature 1] International Publication No. WO03/0197131
[Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2001-143703
[Patent Literature 3] Japanese Unexamined Patent Application Publication No. 2001-143708
[Patent Literature 4] Japanese Unexamined Patent Application Publication No. 2004-103566
[Patent Literature 5] Japanese Unexamined Patent Application Publication No. 2003-7299
[Patent Literature 6] Japanese Unexamined Patent Application Publication No. 2003-331846
[Patent Literature 7] Japanese Unexamined Patent Application Publication No. 2004-348981
[Patent Literature 8] Japanese Unexamined Patent Application Publication No. 2003-20229